Coherent Imaging with Two-Dimensional Focal-Plane Arrays: Design and Applications (Abstract)
Most existing infrared (IR) cameras are incoherent, using the intensity of a scene's blackbody radiation to map thermal distributions over the full bandwidth of a detector array. We are developing a coherent infrared imaging camera (CIRIC) that uses optical heterodyne techniques to determine target intensity and phase information over a narrow frequency interval.1, 2 The technology is based on new developments in two-dimensional quantum-well infrared photodetectors (QWIPs) and HgCdTe detector arrays as well as new system design concepts for optical heterodyne imaging. A schematic diagram of the coherent camera is shown in the figure below. CIRIC consists of a staring 2-D photodetector array illuminated by a specially-formed local oscillator laser beam. Blackbody radiation (passive mode) or scattered laser light (active mode) from a target scene is combined with a local oscillator (CO2 laser) beam on a partially-transmitting beamsplitter element. These overlapping patterns are imaged onto the photodetector array where optical mixing takes place. Analysis of this signal yields information on target parameters such as temperature, range, velocity and spectral composition.
Several recent technology developments are now paving the way for imaging coherent 10 µ lidar systems that use 2-D staring arrays. HgCdTe detectors have historically been difficult to mass manufacture due to material instabilities. Consequently, process yields have been low and HgCdTe detectors have been expensive. As a result of advances in the materials and processes used in fabrication of HgCdTe devices and advances in device configurations, 2-D staring arrays of HgCdTe detectors as large as 480 x 640 are now becoming available.3 In addition, new GaAs/AlxGa1-xAs quantum well infrared photodetector (QWIP) technology is providing high-yield 2-D infrared staring arrays with the added promise of very wide bandwidth heterodyne detection (to 26 Ghz).4 Progress is likewise being made in the minaturization and mass manufacturing of high bandwidth integrated circuits which can operate at liquid nitrogen temperatures. These electronics are fabricated using GaAs-based monolithic microwave integrated circuit (MMIC) technology. MMIC technology is a special case of standard integrated circuits (ICs), optimized for linear and rf applications above 1 Ghz. Typical commercially-available processes allow applications up to about 20 Ghz and some experimental GaAs processes have reported frequencies above 100 Ghz,.5, 6
Currently, the CIRIC project is an internally-funded initiative at the Oak Ridge National Laboratory under the Laboratory Directors' R&D Program. We are developing a low resolution 8x8 optical heterodyne imaging system and its associated high speed electronics to perform laboratory and atmospheric experiments that demonstrate the principles of imaging coherent lidar. To illustrate some of the results to date, an experiment was performed using an open bottle containing NH3, placed adjacent to a blackbody source. Figure 2 below contrasts the resulting video and heterodyne images of the NH3 bottle with and without the bottle cap in place. The NH3 plume is only evident in the upper right hand picture which is a heterodyne image of the open NH3 bottle. Passive heterodyne imaging against a room temperature background has also been demonstrated (where the NH3 plume is heated slightly above ambient) as well as active heterodyne imaging to extract blade velocity in a rotating squirrel cage fan.
Applications of the CIRIC technology include ranging, wind velocity measurements, hard body target characterization (range and velocity), and spectroscopic characterization of atmospheric components (e.g. ozone, volatile hydrocarbons, ammonia, etc.). The unique features of CIRIC are based on the ability to image a scene in 3-D. Each pixel of the camera is an independent laser radar system. This characteristics makes it possible to produce 3-D images of smoke stack plumes, wind fields, and atmospheric constituent fluxes. Traditionally in the 10- µ spectral region, these applications have used single detector, scanning receivers due to the limited availability and cost of array detectors for multiple receiver configurations. Much effort has been expended designing scanning hardware and algorithms to overcome limitations in target acquisition and multiple target or complex pattern tracking via single channel heterodyne detection. With the present advances in detector and electronics technology as well as technical advances made under the present CIRIC project, coherent imaging is now a viable technology.